Accurate Prediction of Enthalpies of Formation of Organic Azides by Combining G4 Theory Calculations with an Isodesmic Reaction Scheme

Abstract

Accurate gas-phase enthalpies of formation (Δf H 298 °) of 29 azides are recommended by combining G4 theory calculations with an isodesmic reaction approach. The internal consistency over a set of Δf H 298 ° values was achieved by sequential adjustment of their values through the isodesmic reactions. The HN3 was chosen as a key reference compound. Of the experimental data available for 16 compounds, our predictive values agree well with 9 of them, while the deviations from 25 to 55 kJ/mol are observed for 7 compounds; possible systematic errors in the experimental data for phenyl azide, 2-azidoethanol, azidocyclopentane, azidocyclohexane, 3-azido-3-ethylpentane, 2-azido-2-phenylpropane, and 1-azidoadamantane are discussed. The recommended enthalpies of formation of organic azides were used as reference values to estimate the enthalpy of formation of four nitrogen-rich carbon nitrides. The calculations do not support the high value of the solid-state enthalpy of formation of TAAT (4,4′,6,6′-tetra­(azido)­azo-1,3,5-triazine); its value is estimated to be 300–400 kJ/mol lower than that measured experimentally.

Enthalpy of formation of guanidine and its amino and nitro derivatives

“…For isodesmic reaction, the resulting enthalpy of formation was calculated combining the G4 enthalpy of reaction with the enthalpies of formation of reference molecules. The accuracy of experimental D f H 298 values for reference species involved in isodesmic and other balanced reactions was supported by G4 calculations applied to atomization reactions in our previous works [23][24][25].…”

“…A good agreement with experiment was obtained only when the isodesmic reactions were used instead of the atomization reaction. The isodesmic reactions were also successfully applied for estimation of enthalpies of formation of organic azides [24] and cyclic nitramines [25,26]. Because of this, the same approach was used in the present study.…”

Thermochemistry of organic azides revisited

“…Using Eqs. (2) and (3) we adjusted vaporization enthalpies from this work [16] to the reference temperature for comparison with our new results: 1- Table 3). These literature results are significantly higher in comparison to our more reliable results from transpiration method (see Table 3).…”

“…Thermochemical data for azides are in disarray [2]. Most of enthalpies of formation and enthalpies of sublimation/vaporization available in the literature are of technical quality or reported without any sample purity information.…”

Enthalpy of formation of CL-20

“…Earlier the same approach was used to obtain the accurate 298 f H Δ (g) values of organic azides [25] and amino acids [26]. …”